Many of image-taking apparatuses are capable of changing the focal length by telescopic motion of their lens barrel in the optical axis direction. In such image-taking apparatuses, their normal operations are required even if the lens barrel receives an external force by hitting of the lens barrel protruded from the apparatus body against objects and person's bodies or by dropping of the apparatus on the ground when carrying the apparatus.
Many of telescopic lens barrels are constituted by a cam barrel provided with a cam groove portion, a lens holding barrel holding a cam follower which engages with the cam groove portion, and the like; the cam barrel and lens holding barrel move relatively. If the above-mentioned external force acts on such a lens barrel, there is a possibility that it cannot be driven because of disengagement of the cam groove portion and the cam follower.
As a measure against this, a lens barrel capable of preventing disengagement of a cam follower from a cam groove portion is disclosed in Japanese Patent Laid-Open No. H06-250063. In this lens barrel, when the lens barrel receives an external force in the optical axis direction, the cam follower biased to the cam groove portion is pressed along an inclined cam surface of the cam groove portion, and a cam follower holding portion of a lens holding frame contacts the cam surface of the cam groove portion, thereby preventing the disengagement of the cam follower and the cam groove portion.
In addition, another lens barrel capable of preventing disengagement of a cam follower from a cam groove portion against an external force in the optical axis direction is disclosed in Japanese Patent Laid-Open No. 2003-15013. In this lens barrel, two inclined surfaces having different angles are provided on the cam follower, and two cam surfaces corresponding to the inclined surfaces are provided on the cam groove portion.
However, the structures disclosed in the above-mentioned publications tend to invite increases in size and production costs of the lens barrel because they require a plurality of parts and a complicated configuration in each of the cam follower and cam groove portion, and in the circumference thereof.
In addition, there is a possibility that the cam barrel or cam follower holding member displaces itself outward in its diameter direction, that is, a direction orthogonal to the optical axis by slipping between the inclined surfaces of the cam follower and cam groove portion in a case where the external force acts on the lens barrel in the optical axis direction. In this case, it is not possible to prevent the disengagement of the cam follower from the cam groove portion in the structures disclosed in the above-mentioned publications.
Furthermore, though the structure disclosed in Japanese Patent Laid-Open No. 2003-15013 is effective for the external force from the front (object side) of the lens barrel, it is not effective for the external force from the rear (camera body side). The image-taking apparatuses are often used with a strap. However, there is a possibility that a pulling force acts on the lens barrel from the rear when a user pulls the strap up at the moment of dropping of the image-taking apparatus with the strap in hand.